Size reduction of metal-oxide-semiconductor field-effect transistors (MOSFET) has enabled the continued improvement in speed performance, density, and cost per unit function of integrated circuits. One way to improve transistor performance is through application of stress to the transistor channel region. Stress distorts (e.g., strains) the semiconductor crystal lattice, and the distortion, in turn, affects the band alignment and charge transport properties of the semiconductor. By controlling the magnitude of stress in a finished device, manufacturers can increase carrier mobility and improve device performance.
One approach of introducing stress into the transistor channel region is to incorporate carbon into the region during the formation of the region. The carbon present in the region affects the semiconductor crystal lattice and thereby induces stress. However, the quality of epitaxially-deposited films decreases as carbon concentration within the film increases. Thus, there is a limit to the amount of tensile stress which can be induced before film quality becomes unacceptable.
Generally, carbon concentrations above about 1 atomic percent seriously reduce film quality and increase the probability of film growth issues. For example, film growth issues such as undesired polycrystalline or amorphous silicon growth, instead of epitaxial growth, may occur due to the presence of carbon concentrations greater than 1 atomic percent. Therefore, the benefits that can be gained by increasing the tensile stress of a film through carbon incorporation are limited to films having carbon concentrations of 1 atomic percent or less. Moreover, even films which contain less than 1 atomic percent carbon still experience some film quality issues.
Therefore, it would be desirable to provide processes that can produce a carbon-free, high tensile stress epitaxial film with good selective growth rate.